The present invention relates to a control system for a compressor driven by a pole-changing motor whose speed is changed to change the compression capacity of the compressor depending upon the variation on load, and more particularly a control system which stops the motor and hence the compressor in case of the pole changing before the motor is started again, thereby ensuring the reliable and dependable pole changing operation.
There have been devised and demonstrated compressors of the type whose fluid delivery capacity may be changed by the change in speed of a motor for driving the compressor in response to the variation in load. For instance, in pole-changing-motor-driven compressors, the windings of the motor are changed to either the high- or low-speed connection depending upon the load on the compressor so that the latter is driven at a high or low speed. However, during the switching between the high- and low-speed connections the power supply to the motor is interrupted for some time which is very short. In general the load torque of the compressor is high while the inertia is small so that when the motor is de-energized the torque produced by the motor immediately decreases lower than the load torque on the compressor and is stopped. Therefore in practice it has been very difficult to attain the continuous pole changing of the motor for driving the compressor. Especially when the winding of the pole-changing motor are changed to the low-speed connection (or full-pole connection) from the high-speed connection (or half-pole connection) and if the motor has been rotating at a speed higher than the synchronous speed at the low-speed connection, the reverse torque is produced, thus resulting in the sudden change in rotational speed of the motor so that the abnormally high stresses are produced in the driving shaft of the motor. To overcome this problem, the motor must be stopped once in case of the pole changing, and then re-started. However, because of the difference in pressure between the suction and discharge sides of the compressor (to be also referred to as "the pressure difference across the compressor" in this specification), the starting load is generally higher than the starting torque of the motor. Therefore the motor must be started again only after the pressure difference across the compressor decreases to a certain level so that the starting load becomes equal to or less than the starting torque available. In other words, the motor must be stopped in case of the pole changing and must be started again a predetermined time after it was stopped. For this purpose, there has been proposed to use time-delay relays or the like, but the conventional time-delay relays or the like must remain on even after the motor is started again in order to keep it driving. More suitable switches such as a pressure-responsive switch which can establish or interrupt the motor driving circuit in response to the pressure difference across the compressor so that the switch is turned off after the motor is once started, have never been used in the control systems of the compressors.